The present invention relates to a method and a circuit arrangement for monitoring a firing circuit for a safety device in a vehicle, the ohmic resistance of a firing element connected in series to a capacitance being measured by charging the capacitance with a measured current for a specifiable charging time, by recording a first firing circuit voltage which drops at the firing circuit at the end of the charging operation and recording a second firing circuit voltage after the measured current is disconnected and by determining the resistance of the firing element from the two firing circuit voltages and the measured current.
Such a method follows from the still unpublished German Patent Application No. 1 98 02 042. In order to ensure that the firing circuit of a safety device (e.g. airbag, belt tightener) functions perfectly in the event of a collision, the firing circuit should be monitored continuously. A fault in the firing circuit is displayed optically or acoustically in the vehicle. In practice, the following faults can occur, for example, in the firing circuit: a short-circuit can arise between the firing element and the battery voltage or ground. In addition, shunt circuits can occur between the feeds of the firing circuit itself or parallel to the firing circuit capacitor. All of these faults are revealed in that the ohmic resistance of the firing circuit deviates from a known setpoint. For that reason, according to the older German Patent Application No. 1 98 02 042, the ohmic resistance of the firing, element is measured and checked for the presence of a setpoint deviation at periodically repetitive intervals. Since in this connection, the resistance of the firing element is derived from voltages which result from various charge states of the capacitance connected in series to the firing element, the resistance measurement depends very heavily on capacitance fluctuations which, for example, are related to manufacture or arise as the result of varying temperature influences.
The object of the present invention is therefore to specify a method and a circuit arrangement of the type named above with which a precise measurement of the resistance of the firing element is possible regardless of fluctuations of the capacitance present in the firing circuit.
The named object is achieved, in that the charging time for the capacitance in the firing circuit is rated in such a way that the second firing circuit voltage, which is measured briefly before the measured current is disconnected, corresponds to a setpoint value. If the capacitance is always charged long enough that the second firing voltage measured after the measuring current is disconnected assumes a fixed value, then the ohmic resistance of the firing element can be measured precisely without fluctuations of the firing circuit capacitance having an influence on it.
Accordingly, the charging time for the two firing circuit voltages is determined before the measurement operation by charging the capacitance for a fixed basic charging time after which the measured current is disconnected and the voltage which drops at the firing circuit is then measured. This measured voltage is compared with a setpoint voltage and a correction charging time is determined as a function of the difference between these voltages, a charging time being determined from the correction charging time by addition to the basic charging time, which ensures that the second firing circuit voltage measured in the subsequent measurement operation corresponds to the setpoint value.
This measurement operation can be implemented with a simply configured circuit which requires no expensive current source for the charging of the capacitance in the firing circuit.